Non-Iridescent Structural Color Control <i>via</i> Inkjet Printing of Self-Assembled Synthetic Melanin Nanoparticles

Melanin is a natural pigment with a high refractive index and strong light absorption across the visible spectrum, making it an ideal material for producing structural colors. Here, we report non-iridescent structural color control via inkjet printing of self-assembled synthetic melanin nanoparticles (SMNPs). Adding silica shells to SMNPs allows for further tuning of both the hue and brightness of the resulting structural colors. The peak wavelengths show a linear dependence with the diameter of the nanoparticles, allowing correlation between ink composition and structural color using the Bragg–Snell law. Additionally, mixtures of SMNPs of different sizes result in colors with peak wavelengths that vary linearly with the mixing ratio in the ink, leading to diverse and predictable colors from one type of material. The morphology of the self-assembled SMNP structures is further controlled by the hydrophilicity of the substrate, providing another means for tailoring the structure and properties. Since structural colors are less susceptible to degradation than organic dyes, this work has implications for emerging sensing, display, and security applications